This invention relates to an information recording medium. The information recording medium has an optical information recording layer. A laser beam is focused on the recording layer to form a pit therein, thereby recording optical information. The optical information recorded in this fashion can be read out. Alternatively, the information recording medium has a light reflective layer having optical information recorded therein in the form of, for example, patterned recesses and projections. A laser beam is focused on the reflective layer so as to read out the recorded information from the light reflected from the reflective layer.
Recently, the information recording medium permits a high density recording. For example, the recording density achieved nowadays is scores of times to one hundred times as high as that of a magnetic memory. Specifically, the information can be recorded at a track width of 1 .mu.m or less and a track pitch of 2 .mu.m or less and the information recorded at such a high density can be reproduced. Also, a single disk, 30 cm in diameter, of the information recording medium permits achieving a capacity of 50,000 tracks and 10.sup.10 bits. The information recording medium produces additional merits including the following:
1. Information can be recorded and reproduced in a non-contact fashion, PA1 2. Random access is permitted at a high speed, PA1 3. The medium can be preserved stably for a long period of time, PA1 4. Recording system is simple, PA1 5. Additional information can be recorded, and PA1 6. The cost per bit is low.
Because of these merits, the information recording medium is widely used for the information recording-reproducing purpose or for the reproducing purpose alone. The information recording medium for the reproducing purpose alone is used in, for example, a domestic video disk, a video disk for business, and an audio PCM disk. On the other hand, the information recording medium for the recording-reproducing purpose is used in, for example, a document file, a video file and a computer general purpose memory.
The conventional information recording medium used in an information processing device of this type comprises two substrates each formed of, for example, a transparent plastic plate. Formed directly on one surface of each substrate is an optical information recording layer consisting of an energy-absorbing layer or a light reflective layer. The two substrates of this construction are disposed to face each other such that the recording layer is positioned inside, and are bonded to each other with spacers disposed therebetween so as to provide a free space therebetween. Alternatively, the two substrates are directly bonded to each other, with an adhesive coated on the entire surface of at least one substrate.
The conventional information recording medium of the construction described above is defective in that the transparent plastic substrate is high in permeation rate of water or oxygen. Thus, the oxygen or water contained in the atmosphere readily permeates through the substrate to enter the optical information recording layer. It follows that the deterioration of the recording layer is promoted, leading to reductions in the light reflecting rate and light absorbing rate and, thus, to a low sensitivity. In addition, the recording layer tends to peel off the substrate.